Supported liquid membranes have been known in the art for several decades as a potential method for the separation of various permeants, being a gas or solute, from a feed-fluid mixture. These liquid membranes usually involve the incorporation of a membrane solution into the micro-pores of a porous membrane. The membrane draws up the solution into its pores so that the pores act as a selective membrane for selective separation of the permeant from a feed-fluid. That is, the membrane solution is selectively chosen so that the permeant will pass through the solution in the micro-pores thereby separating the permeant from the feed-fluid.
The geometry of the microporous support membrane can be flat sheet, hollow fiber and spiral-wound. Membrane modules containing the above three geometries are commercially available. A flat sheet module is less attractive due to its low membrane area/volume ratio. Hollow fiber and spiral-wound modules are more desirable because they have a higher membrane area/volume ratio and have been used in many industries for gas separations, ultrafiltration and reverse osmosis, etc. They can also be used to form the supported liquid membranes by incorporating membrane liquid in their pores.
There are numerous attempts to use conventional hollow fiber modules for liquid membrane applications. The conventional spiral-wound modules used in reverse osmosis and ultrafiltration cannot be used due to their structural limitations. A certain modified spiral-wound module is amenable to the liquid membrane applications. For example, U.S. Pat. No. 5,034,126 to Reddy et al. teaches a countercurrent dual-flow spiral-wound structure. This structure has a feed pipe separated into compartments which are axially connected to a porous spacer surrounded by a membrane envelope. A separate permeate pipe is immediately adjacent in a parallel configuration to the fluid feed pipe and is in fluid communication with a different porous spacer sheet. A feed stream containing a permeant is forced through the module through the feed pipe. As the feed stream moves through the spiral-wound membrane, the permeant passes through the membrane into the permeant passageway. The permeant is removed from this passageway by means of a sweep fluid.
Although these hollow fiber and modified spiral-wound structures provide a useful solution to the problem of limited membrane surface area, their commercial applications are prohibited due to the common problems, the loss and degradation of the membrane solutions in the pores, which cause performance instability and short life span of conventional supported liquid membranes.
There are extensive efforts to improve the supported liquid membrane's stability and reliability. For example, U.S. Pat. No. 4,750,918 to Sikar teaches a hollow-fiber-contained liquid membrane structure in which two independent sets of hollow fibers are incorporated in a shell housing. A selective-permeation liquid(membrane solution) is introduced into the shell housing to form a permeation transfer chamber. One set of the fibers serves as a gas-depletion channel and the other as a gas-enrichment channel. Both channels pass through the permeation chamber and are separated by the liquid in the chamber. The selective-permeation liquid in the permeation chamber contacts the porous walls and facilitates the transfer of a selected gas from a feed-gas mixture in the gas-depletion channel to the gas-enrichment channel. A similar structure is taught in U.S. Pat. No. 4,789,468, also to Sikar, which incorporates pressure regulators between a feed and solution channel and an extractant channel to substantially immobilize the interface between the channels for liquid separations.
Although these unique liquid membrane structures are theoretically sound and have better stability by replenishing the lost membrane solution in the permeation chamber, several problems have been encountered in connection with adapting them for large-scale practical use. For instance, the structural characteristics of the membrane channels are not well defined. It is difficult to obtain uniform inter-distributions of the permeant depletion and enrichment channels in the permeation chamber, resulting in the non-uniform effective liquid membrane thickness and permeability. Furthermore, the permeability can be relatively low due to the relatively thick and stagnant liquid between the depletion and enrichment channels.
Accordingly, attempts have been made to increase permeability by creating a continuously flowing membrane liquid, thereby generating three independent and simultaneously flowing fluid channels. This is called a moving liquid membrane system. One example of such a system is provided in an article by Teramoto et al. in the Journal of Membrane Science, 45 (1989) 115-136 (Elsevier Science Publishers B.V., Amsterdam). Teramoto teaches a flat sheet configuration in which a liquid membrane solution flows in a thin channel between two spaced microporous membranes. Although increased permeability is achieved, the membrane area limitations discussed above have limited the use of such a structure in commercial applications.
In view of this, moving liquid membrane structures have been incorporated into spiral wound modules. An example of a spiral wound moving liquid membrane structure is described in an article by M. Teramoto et al. in Separation Science and Technology, 24(12 & 13), pp. 981-999, 1989. Teramoto discloses a spiral wound structure in which the liquid membrane solution containing a carrier flows in thin spiral wound channels between two hydrophobic microporous membranes which separate the membrane solution from a feed and a strip solution. Again, however, this structure is not amenable to commercial application since it is difficult to fabricate into a unit with industrially acceptable through-put.